1. Field of the Invention
The present invention relates to a method and apparatus for measuring impedance. More specifically, the invention relates to an impedance measuring or determining apparatus which is incorporated in and used in a neural stimulating system where it is desirable to monitor and determine changes in a cathode lead impedance in a lead connected to a cathode implanted in the epidural space in a spine.
2. Description of the Prior Art
Heretofore, various apparatus for supplying neural stimulation pulses to nerves in the spine have been proposed. These prior neural stimulating devices and systems have also provided means for modifying, molding, patterning and altering the current signals supplied to the cathode electrode. Examples of these previously proposed neural stimulating systems and devices are disclosed in the following U.S. patents:
______________________________________ U.S. Pat. No. PATENTEE ______________________________________ 3,769,986 Herrmann 3,918,459 Horn 3,989,051 Nozhnikov et al. 4,068,669 Niemi 4,088,141 Niemi 4,102,347 Yukl 4,167,189 Tachi et al. 4,338,945 Kosugi et al. 4,354,498 Weigert et al. 4,390,023 Rise ______________________________________
An examination of these prior art patents will show that none of them disclose or suggest a lead impedance measuring system wherein pulses of a given amplitude are compared with a ramp voltage and wherein the given amplitude is related to the impedance then existing in a lead or circuit being monitored.
The Rise U.S. Pat. No. 4,390,023 merely discloses production of a patterned stimulus through multiple cathodes of a Transcutaneous Electrical Nerve Stimulator (T.E.N.S.).
The Weigert et al. U.S. Pat. No. 4,354,498 merely discloses a constant current method of measuring current across a known series resistance.
The Kosugi et al. U.S. Pat. No. 4,338,945 merely discloses randomizing of stimulus parameters and uses a microprocessor for this purpose.
The Nozhnikov et al. U.S. Pat. No. 3,980,951 merely discloses an external device similar to a T.E.N.S. having adjustable duration and amplitude.
The Herrmann U.S. Pat. No. 3,769,986 merely discloses a threshold analyzer using a current control device to determine-tissue thresholds using a controlled pulse current.
The Horn U.S. Pat. No. 3,918,459 merely discloses a constant current electrotherapy device which utilizes a DC signal and an alarm for high resistance and appears to be directed to an external device as opposed to an internally implanted device.
The Niemi U.S. Pat. No. 4,068,669 is directed to a stimulator fault protection circuit where the output current is monitored and the circuit limits high currents.
The Tachi et al. U.S. Pat. No. 4,167,189 is directed to an apparatus for transmission of information by electrocutaneous stimulus and measures I.sub.0 and V.sub.0 and then calculates Z=V.sub.0 /I.sub.0. Here, the apparatus is directed to means for transmission of data and not to a technique for measuring.
The Niemi U.S. Pat. No. 4,088,141 discloses a fault circuit for a stimulator in a T.E.N.S. versus an implantable system. In this stimulator, a threshold impedance is monitored using a voltage divider in a time to voltage conversion within a microprocessor. This stimulator utilizes a voltage comparator and a constant current source. However, it does not compare voltage pulses required to maintain constant current pulses with a ramp voltage for the purpose of determining impedance as provided with the method and apparatus of the present invention.
Finally, the Yukl U.S. Pat. No. 4,102,347 discloses an electrical pain control system which monitors impedance threshold and limits a voltage applied if the impedance is too high. This system operates in a manner very similar to the stimulators disclosed in the Niemi patents referred to above.
As will be described in greater detail hereinafter, the method and apparatus of the present invention differ from the previously proposed methods and apparatus for neural stimulation which include some form of impedance monitoring circuitry by providing an impedance determining apparatus which includes a comparator for comparing an increasing ramp voltage with voltage pulses necessary to maintain constant current pulses supplied to the impedance.